It is the purpose of the present invention to radically increase the load-bearing capacity and durability of journal bearings for reciprocating machinery, particularly gasoline and diesel engines, and at the same time reduce bearing friction loss.
Engine main and connecting rod bearings have been design-limiting and a source of engineering concern since the early days of engines. Significant efforts have been made to improve engine oils and bearing materials to tolerate the marginal lubrication conditions under which engine bearings operate. This work is costly and at best treats only the symptoms of the bearing design problem. Several billion dollars worth of petroleum per year is wasted by excess friction loss in engine bearings which perform well below their theoretical limits.
Although engine oils have been greatly improved, the limits of engine bearing load capacity and friction characteristics have not been conspicuously improved since World War II. In World War II men frequently died in air combat because their engine power was not as great as it needed to be for them to outmaneuver their enemy. Airplanes were often lost from bearing failures due to combat overloads. For this reason, the large number of excellent mechanical engineers, then working to continuously improve combat engine performance, were under intense pressure to increase main and connecting rod journal bearing performance, and they did so. The structural, hydrodynamic, and material compatibility characteristics of bearings received more intense scrutiny than they have since. Nonetheless, bearings continued to significantly limit engine performance until the end of the war, and continue to do so.
Well before World War II, experience had formed an engineering consensus that powertrain journal bearings would operate under such severe conditions that surface rubbing contact between bearings and journal was inevitable; therefore, research and development has focused on the boundary lubrication characteristics of lubricants, the ability of bearing materials to run against the crankshaft journal without welding or scoring, the corrosion resistance and fatigue strength of bearing materials, and manufacturing techniques to conserve the somewhat costly bearing materials used. Boundary lubrication is a very difficult field and theoretical predictions are not very trustworthy. The development of bearings and lubricants has therefore been largely empirical, with the empirical work guided by very sophisticated knowledge of the molecular-level physical and chemical characteristics of boundary lubrication.
The current excellent maintenance record of engine bearings is due to precise knowledge of bearing limitations and design of machines within these limitations. When these limitations are pushed, for example in racing engines, bearings fail. Bearing limitations are also the most basic limit on peak cylinder pressures in diesel engines (although current cylinder head sealing would have to be improved to take advantage of increased bearing load capacity). Significant fuel economy and power improvements would be possible if these peak pressures could be increased.
An interesting contrast to the connecting rod big end bearing, which has long been a source of engineering attention, is the wrist pin bearing joining the connecting rod to the piston. These bearings almost never fail, and have been essentially trouble-free throughout the history of engine development. They typically operate at pressure loads twice that of the big end journal bearings, and their wear characteristics indicate that they almost always operate in a full oil film condition without surface contact. For instance, steel on steel rubbing contact in boundary lubrication very often leads to contact welding and bearing seizure, but steel wrist pins bear directly on steel surfaces in the connecting rods of production engines without difficulties. The wrist pin bearing's full oil film performance is at first sight the more surprising because of its low sliding speed. Sliding speed acts to build up a hydrodynamic oil film. It is an interesting commentary on the connection between the generally empirical tradition of engine design and the mathematical and physical tradition of lubrication that wrist pins in steam and automotive engines took loads much higher than predicted by the theory for about 100 years until Professor Dudley D. Fuller of Columbia University explained their function by the squeeze film effect in the 1950's.
Under the present invention load capacity and film stability similar to that already shown by wrist pin bearings and much lower bearing friction loss than current levels is obtained with simple part geometry. Data and theory both indicate that it should be possible to build connecting rod bearings and main bearings which are essentially trouble-free and operate with low friction loss at the highest loadings to which engine structures can plausibly be built - peak bearing loadings in excess of 20,000 lbs/in.sup.2 of bearing projected area. Similar designs should be applicable to forging presses and other very highly loaded bearing applications at full film bearing loads approaching or exceeding 40,000 psi. This is radically better bearing performance than has existed in the past.
To achieve this greatly enhanced load-bearing capacity and to reduce friction requires that two things be done simultaneously. First, the difference in radii of curvature (mismatch) of the rotating journal and the bearing surface must be sufficiently large (very much larger than conventional practice) so that the parts, in mechanical interaction with the oil film under load deform in a manner so that the system produces proper relative geometric characteristics. The important geometric relationships between bearing and journal must exist under load rather than in the static condition. Recognition of the interaction between elastic deformation and the hydrodynamic fluid mechanics in the bearing results in radically different tolerancing of the relatively sliding bearing surfaces than common practice. For the invention it is typically necessary to arrange the geometry so that at least two arcs of the bearing circumference have different centers of curvature to make the large radius of curvature differences between journal and bearing required, without excessive mechanical freedom of the bearing with respect to the journal. The differences in center of curvature typically produce a very beneficial effect of smoothly closing voids in the oil film caused by cavitation, prior to the oil film entering the heavily loaded high pressure arc of the bearing. Second, it is necessary to arrange sealing means, which could be accomplished by an O-ring arrangement or by relatively tight axial clearances between journal and bearing at the side, so that the bearing stays full of oil and does not allow the fluctuating forces and displacements of bearing operation to pump the bearing dry. Restricting the exit of oil from the bearing so that the bearing operates full of oil produces much thicker and more stable oil films than current practice and essentially eliminates the boundary lubrication problems which have plagued conventional journal bearings in reciprocating load applications.
The current invention exists in such a crowded art that it is important to explain why the invention was not obvious in view of the prior art. The basic reasons for this have to do with an improved understanding of bearing function in the presence of unavoidable structural deflections of the sliding parts under load. In terms of the prior understanding of proper journal bearing design, the bearing of the present invention is designed wrong. However, this "wrong" design functions much better than previously accepted designs.
The body of accepted design practice from which the present invention departs is massive. In many ways, the best explanatory reference to the lubrication arts in the English language is Dudley D. Fuller's Theory and Practice of Lubrication for Engineers, Second Edition, John Wiley & Sons, New York, 1984, which will be cited in a number of places in this application. On pages 262 and 263 of that reference, Fuller states:
" . . . there have been few attempts to digest the great mass of technical literature available on journal bearing design and performance and systematize it for the use of the practicing engineer. Certainly, as a busy man, he does not have the time to do this for himself. For example, on journal bearings alone the American Society of Lubrication Engineers has prepared an annotated bibliography of 563 references with an overall appraisal of their value. They were selected from some 2500 publications through the year 1954. Since then, perhaps another thousand or more papers and books have ben added to the literature. To assemble--in some cases, translate--read, and organize this material would be an impossibility for the busy practicing engineer." PA1 "The charts show the performance variables as functions of the bearing characteristic number or Sommerfeld number, S, where ##EQU2## This number united in one dimensionless variable most of the factors (only the bearing arc .beta. and L/D ratio are omitted) over which the designer has direct control. Its major importance lies in the fact that, once this number has been established, all the operating characteristics (of the bearing) become fixed." PA1 "Typical schematic drawings of tapered lands or of pivoted pads can be very misleading. The slope for good load-carrying capacity is only about 0.003 in./ft, or approximately one unit in 4000 units. Thus, for example, if a bearing pad were as long as a football field (100 yd or 91.47 m), the proper elevation of one end over the other, for good hydrodynamic load-carrying capacity, would be about 0.9 in. (2.28 cm)!" PA1 "Many types of machines produce pulsating or reciprocating loads on bearings and bearing surfaces on which, because of high pressures and lack of continuous sliding or turning motion, oil film breakdown and relatively severe wear are to be expected. Strangely enough this wear does not always appear. Crossheads, piston-pin type bearings, cams, tappets, and knuckle-joint bearings may show remarkable freedom from metal-to-metal contact. Apparently, when conditions are favorable, an oil film is maintained between the contacting surfaces even though the relative motion of these surfaces becomes momentarily zero. PA1 This load-carrying phenomenon arises from the fact that a viscous lubricant cannot be instantaneously squeezed out from between two surfaces that are approaching each other. It takes time for these surfaces to meet and during that interval, because of the lubricant's resistance to extrusion, a pressure is built up and the load is actually supported by the oil film. If load is applied for a short enough period, it may happen that the two surfaces will not meet at all. PA1 When the load is relieved or becomes reversed, the oil film often can recover its thickness in time for the next application, if the bearing has been designed to permit and assist this build-up. Indiscriminate location of oil holes, oil grooves, and reliefs may interfere with the restoration of the oil film and destroy the major part of its load-carrying capacity."
The most basic point of departure of journal bearing analysis is that of A. Sommerfeld, who in 1904 derived the dimensionless group ##EQU1## called the Sommerfeld variable. In the equation, R is the journal radius, c is the difference in radius of curvature between the journal cylindrical surface and the cylindrical surface of the bearing, .mu. is viscosity of the lubricant; N is shaft speed in revolutions per second; and P is the force per unit projected area (force divided by journal radius times journal axial length). In another very good and basic reference in lubrication, Standard Handbook of Lubrication Engineering, James J. O'Connor, and John Boyd, editors, McGraw Hill Book Company, 1968, the conventional wisdom concerning the Sommerfeld variable is stated on pages 5-40 of Chapter 5, written by Raimondi, Boyd and Kaufman.
The journal bearing charts plot bearing performance in terms of the Sommerfeld variable on pages 5-41-44 and 5-49, 5-52, 5-53, 5-54, 5-55 of the Standard Handbook. Almost all journal bearing analysis is based on the mathematical work of Sommerfeld.
A major key to the present invention lies in the recognition that the Sommerfeld variable, as it is conventionally interpreted, is systematically misleading in bearing design and that an understanding of the real physical situation permits great improvements in practical bearing performance. This recognition is not obvious, or bearing problems which have persisted for many years would have been eliminated long ago.
The recognition did not come easily to the inventor, who has for years attempted to improve practical bearing performance. Specifically, the inventor worked and thought intensely about the problem of improving load capacity in bearings and, through a systematic misunderstanding of the problem, developed very high load-bearing capacity pivoted pad bearings and journal bearings set forth in the co-pending applications (Ser. No. 502,468 Hydrostatically Supported Pivoted Pad Slider Bearing with Very High Load-Bearing Capacity, Ser. No. 502,581 Connecting Rod, Ser. No. 502,572 Superposition of Pivoted Pad Slider Bearing and Squeeze Film Fluid Mechanics for Heavily Cyclically Loaded Machine Elements all now abandoned). The inventor's company spent several hundred thousand dollars developing these bearing concepts. It was experimental problems and results from this work which led to the current invention.
To understand the present invention, it is important to have a sense of the scale on which the crucial geometric relations govern the performance of hydrodynamic bearings. These distances and angles are not directly accessible to human senses, yet they are vital. Fuller makes the point vividly on page 209 of the previously cited work in reference to another hydrodynamic geometry, but the point is equally valid for journal bearings.
For heavily loaded journal bearings, the convergence angle between the sliding surfaces in the portion of the bearing of most physical importance may be much less than the 0.003 in/ft cited by Fuller--as little as 0.0003 in/ft or less. It is hard to think about building real hardware to function with geometric relations as fine scale as this.
In addition to the problem of fine scales, the basic equation of hydrodynamic lubrication, Reynolds' equation, is somewhat difficult to manipulate and think about, even for professionals. Virtually all mathematical analyses involve simplifying mathematical assumptions, and these simplified assumptions may make an analysis invalid or misleading. Particularly, all journal bearing analyses of which the inventor is aware assume an infinitely stiff bearing structure--a radically misleading assumption in most bearings, since even very tiny deflections can be significant on the scale at which hydrodynamic fluid mechanics happens. Conventional analysis is done with the assumption that the lubricant (if it is a liquid) is incompressible. This analysis becomes radically incorrect if the oil contains any significant concentration of bubbles, even tiny ones.